Field strength uniformity control system for article theft detection system

ABSTRACT

An electromagnetic interrogation field in a theft detection system is made effectively more uniform by energizing different antenna windings lying in different planes at the same frequency but at different phases so that the resulting field pattern rotates in the vicinity of the antenna windings.

States atent 1 inasy et al.

[ FIELD STRENGTH UNIFORMITY CONTROL SYSTEM FOR ARTICLE THEFT DETECTIONSYSTEM [75] Inventors: Arthur J. Minasy, Woodbury;

Ronald Pru zick, Commack, both of [21] Appl. No.: 351,018

[ Sept. 24, 1974 3,740,742 6/1973 Thompson et al. 340/280 PrimaryExaminer-Glen R. Swann, Ill

Attorney, Agent, or Firm-Fitzpatrick, Celia, Harper & Scinto [5 7]ABSTRACT An electromagnetic interrogation field in a theft de- [52] US.Cl. 340/258 C, 340/280 e i n sy m is made effectively more uniform by[51] Int. Cl. G08b 13/24 energizing different antenna windings lying indifferent [58] Field of Search 340/258 C, 280 planes at the samefrequency but at different phases so that the resulting field patternrotates in the vicinity of [56] References Cited the antenna windings.

UNITED STATES PATENTS 3,500,373 3/1970 Minasy 340/258 R 19 Claims, 11Drawing Figures to 2e 0 fiz Ana/F59 Q- ecroq I r t2 4&1 I I fir/6P7 76mmA? 24- Vsqncm. flow? 0867:0970)? 0504mm 8 -45Bnsc 32 ,4

fig; Awe/flee 275002 3 58 ip/zo/mc I 36 Abe/me H t FIELD STRENGTHUNIFORMITY CONTROL SYSTEM FOR ARTICLE THEFT DETECTION SYSTEM BACKGROUNDOF THE INVENTION 1. Field of the Invention This invention relates toelectronic theft detection systems and more particularly it concernsnovel arrangements for maintaining the vicinity of a theft detectioncheckpoint substantially uniformly filled with an electromagneticinterrogation field.

2. Description of the Prior Art The present invention is especiallysuitable for use in conjunction with electronic theft detection systemsof the type described in US. Pat. Nos. 3,493,955 and 3,500,373. In bothsystems, each of the articles to be protected from theft has aelectronic responder circuit attached to it. This circuit may beconcealed in a wafer like element which may also serve as a price labelor the like for the protected article. The articles are maintained in anenclosure having limited egress and check points are set up at eachegress. A transmitter is provided at the checkpoint to transmit aninterrogation signal and receiver means are provided to note anyresponse produced by the interaction of a wafer responder circuit withthe transmitted signal field in the vicinity of the checkpoint. In thecase of the systems described in US. Pat. No. 3,493,955, the waferresponder circuits respond to the transmitted interrogation signal,which is at first frequency, to produce a response signal at a secondfrequency. The receiver means are tuned to detect this second frequency.

In the case of the system described in US. Pat. No. 3,500,373, the waferresponder circuits are resonant circuits tuned to resonate at thetransmitted interrogation frequency. When these wafer responder circuitsare brought into the transmitted interrogation signal field they absorbsome of the transmitted energy. The receiver means monitors thetransmitted signal, which changes in amplitude due to this absorption.In order to maximize sensitivity the transmitter of this system producesan output frequency which sweeps cyclically over a given range whichincludes the resonant frequency of the wafer responder circuits. Thiscauses a series of responses in the form of impulses which occur at arepetition rate corresponding to the frequency sweep rate.

The ability of a responder circuit to function effectively in anyelectronic theft detection system depends upon the degree to which theinterrogation field is incident upon the responder circuit. Since theseresponder circuits are generally in flat wafer-like form, they exhibitdifferent degrees of sensitivity depending upon their orientation withrespect to the interrogation antenna. In order to accomodate thedifferent attitudes which responder devices may assume when carriedthrough a checkpoint there has been developed a plural antenna systemcomprising at least two antennas positioned at right angles to eachother at the checkpoint. The two antennas are energized simultaneouslyso that as a responder circuit is turned away from one interrogationantenna it turns toward the other interrogation antenna so thatsensitivity is maintained. This plural interrogation antenna system isshown and described in US. Pat. No. 3,493,955.

mal electromagnetic field intensity. If a responder circuit is caused tofollow a path through the dead zone regions of a checkpoint only minimalinteraction will occur between the interrogation signal and theresponder circuit and it is possible that the passage of the respondercircuit through the checkpoint will not be detected.

SUMMARY OF THE INVENTION The present invention overcomes the abovedescribed field distribution problems and provides an interrogationfield in which dead zones are effectively minimized.

According to the present invention there are provided at least twointerrogation antennas at a checkpoint. These antennas are positioned indifferent planes, preferably at right angle to each other. The antennasare energized simultaneously at the same interrogation frequency.However this energization is controlled so that a phase differenceexists in the energization of the different antennas. Preferably thisphase difference is ninety degrees. As a result of the energization ofantennas lying in different planes with signals of different phase,there is produced in the vicinity of the checkpoint a rotatingelectromagnetic field. Thus, while a dead zone may exist in the field atany given orientation thereof, the rotation of the field causes the deadzone to move so that the entire region of the checkpoint is effectivelyfilled with the electromagnetic field. In effect the dead zones areeliminated and the responder circuits are more likely to be detected.

There has thus been outlined rather broadly the more important featuresof the invention in order that the detailed description thereof thatfollows may be better understood, and in order that the presentcontribution to the art may be better appreciated. There are, of course,additional features of the invention that will be described hereinafterand which will form the subject of the claims appended hereto. Thoseskilled in the art will appreciate that the conception upon which thisdisclosure is based may readily be utilized as a basis for the designingof other structures and other methods for carrying out the severalpurposes of the invention. It is important, therefore, that the claimsbe regarded as including such equivalent structures and methods as donot depart from the spirit and scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS A specific embodiment of the inventionhas been chosen for purposes of illustration and description, and isshown in the accompanying drawings forming a part of the specification,wherein:

FIG. 1 is a block diagram of a swept frequency electronic theftdetection system in which the present invention is embodied;

FIG. 2 is a perspective view illustrating the arrangement of antennacoils according to the present invention;

FIG. 3 is a fragmentary circuit diagram showing phase shifting circuitsused in the system of FIG. 1;

FIGS. 4 A-D are diagramatic representations showing magnetic fieldrelationships around portions of antenna coils during antennaenergization at different intervals in a cycle of energization accordingto the prior art; and

FIGS. 5 A-D are views similar to FIGS. 4 A-D but showing magnetic fieldrelationships at different intervals in a cycle of energizationaccording to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The swept frequency theftdetection system of FIG. 1 is, in part, like that shown and described inUS. Pat. No. 3,500,373. As shown, there is provided a tuning oscillatorwhich produces a voltage whose amplitude varries at a given rate, e.g.,300 cycles per second. This varying voltage is applied to a voltagetuneable swept radio frequency oscillator 12 which is designed toproduce an output voltage which is nominally 2 megahertz (MHZ). When theoscillator 12 is controlled by the tuning oscillator 10 however, itsoutput frequency is swept between 1.95 MHZ and 2.05 MHZ at a 300 cycleper second rate.

The swept frequency output of the oscillator 12 is supplied to ajunction 14 (see FIG. 3) from which it branches to lead and lag phaseshifters l6 and 18. These phase shifters, which will be described ingreater detail hereinafter in connection with FIG. 3 act, respectively,to advance and retard the phase of the signals applied to them. In thecase of the lead phase shifter 16, the signal phase is advanced byapproximately 45, while in the case of the lag phase shifter, the signalphase is retarded by approximately 45 degrees. There is thus produced anet phase differential of 90 at the outputs of the two phase shifters 16and 18.

The output of the lead phase shifter 16 is applied to an amplifier 20;and the output of this amplifier is applied to a transverse antennajunction 22. A transverse antenna winding 24 is connected to thejunction 22. The junction 22 is also connected to a first detector 26;and this is in turn connected to an alarm 28.

The output of the lag phase shifter 18 is applied to an amplifier 30;and the output of this amplifier is applied to a pair of lateral antennajunctions 32 and 34.

Each of the detectors 26 and and the alarm 28 may be constructed andarranged as described in connection with FIG. 2. As is there shown, thetransverse antenna winding 24 comprises a multiturn coil lying in aplane which is substantially perpendicular to a path of egress(indicated by an arrow A) through a checkpoint. Various physical means(not shown) are provided to confine the movement of protected articles,which are equipped with electronic responder circuits, so that they canexit from an enclosure only via the path shown by the arrow A. Theprotected article thus must pass either through or very near thetransverse antenna winding 24 during egress from the enclosure.

The two lateral antenna windings 36 and 38 also comprise multiturncoils. However these antennas are arranged on opposite sides of theegress path with their planes oriented parallel to the path andperpendicular to the plane of the transverse antenna winding 24.

As shown in FIG. 2, all of the antenna windings have one end connectedto ground. The opposite end of the transverse antenna winding 24 isconnected to the junction 22 while the opposite end of the lateralantenna windings are connected to the junctions 32 and 34.

The system of FIGS. 1 and 2 operates to detect the presence of resonantresponder circuits (not shown) carried on protected articles which passthrough and by the antenna windings 24, 36 and 38 along the egress path.The responder circuits are tuned to resonate at a frequency within thesweep range of the oscillator 12. Thus the responder circuits may betuned to resonate at 2 MHZ.

When no responder circuit is present in the egress path, a relativelyhigh impedance is presented to the antenna windings and most of theenergy from the amplifiers 20 and 30 passes by the junctions 22, 32 and34 and becomes incident upon the detectors 26 and 40. As long as arelatively constant energy level is applied to the detectors they do notproduce any alarm actuating signal.

When a responder circuit passes by or through the antenna windings, itresonates and absorbs energy each time the frequency of the oscillator12 sweeps by the resonant frequency of the responder circuit. When theresponder circuit is tuned to resonate at 2 MHZ and the oscillatorfrequency sweep between 1.95 MHZ and 2.05 MHZ, this resonant responseoccurs twice during each sweep cycle or at a 600 response per secondrate. The resonant responses cause a decrease in impedance in thevicinity of the antenna windings so that more of the transmitted energypasses out from the windings during the resonant response. This resultsin a decrease in the energy level applied to the detectors 26 and 40.The detectors, as described in US. Pat. No. 3,500,373, are provided withspecial arrangements for detecting these energy decreases and foractuating the alarm 28 when they occur.

The phase shifters 16 and 18 may be of any suitable construction whichwill produce a difierence in phase between the lateral and transverseantenna windings. The particular phase shifter construction shown inFIG. 3 has been found to be quite suitable for the present application.As shown in FIG. 3, the lead phase shifter 16 comprises a resistor 42connected in series between the junction 14 and the amplifier 20. Acapacitor 44 is connected across the resistor 42. The lag phase shifter18 comprises a resistor 46 connected in services between the junction 14and the amplifier 30. An inductor 48 and a capacitor 50 are togetherconnected across the resistor 46. It has been found that when theresistor 42 is about 680 ohms and the capacitor 44 is about picofaradsthe lead phase shifter 16 will produce a phase shift of close to +45 forfrequencies in the range of 1.95 2.05 MHZ. Also when the resistor 46 isabout 680 ohms and the inductor 48 is about 47 microhenries, the lagphase shifter will produce a phase shift of close to 45 for frequenciesin the range of 1.95 2.05 MHZ. The capacitor 50 is used to prevent shortcircuiting of the resistor 46 by the inductor 48; and it has been foundthat this capacitor will serve this function without adversely affectingphase shift for the frequencies mentioned when its capacitance isapproximately 0.1 microfarad.

The phase shifters l6 and 18 may be adjusted to produce any differentialin the net phase shift between the lateral and transverse antennawindings. However, as will be seen, a 90 net phase shift should producea more uniform field distribution with the antenna arrangement of FIG.2. Also, it is not necessary that two separate phase shifters be used. Asingle phase shifter capable of producing a 90 phase shift in the signalto one of the amplifiers 20 or would produce a similar result. The useof two phase shifters, each of which produces only a 45 phase shift,however, permits a more accurate phase shift over the frequency sweeprange with less expensive construction. Also it has been found that thefrequency sensitivity of the two phase shifters l6 and 18 iscomplimentary during the 1.95 to 2.05 MHZ frequency sweep. Thus when thefrequency at any instant is such that the lead phase shifter 16 producesless lead, that same frequency causes the lag phase shifter 18 toproduce greater lag so that the net phase difference remains essentiallythe same as frequency shifts.

The manner in which the above described antenna orientation and phaseshifted antenna excitation serves to produce a revolving field whicheliminates dead zones can be seen in a comparison of FIGS. 4 and 5.FIGS. 4 and 5 each comprise a group of section views taken along line 44of FIG. 2. These section views sever the vertical portions of both thetransverse and horizontal antenna windings 24, 36 and 38. Thus there areshown severed ends of vertical portions 24a and 24b of the transverseantenna winding 24, severed ends of the vertical portions 36a and 36b ofthe lateral antenna winding 36 and severed ends the vertical portions38a and 38b of the lateral antenna winding 38. As can be seen in FIG. 2,when current flows upwardly in the vertical portion 24a, it flowsdownwardly in the opposite vertical portion 24b. This also applies tovertical portions 36a and 36b and 38a and 38b. Reverse current flowsoccur, of course, during one half of each cycle of antenna energization.

The flow of current through the various antenna windings is accompaniedby a circular magnetic field surrounding the winding wires asillustrated by arrows B in FIGS. 4 and 5. The direction of the variouscircular magnetic fields B corresponds to the direction of current flowthrough the winding which each field surrounds. Thus, the direction ofthe circular magnetic field reverses itself upon each half cycle ofantenna energization.

The distribution of electromagnetic energy in the vi- I cinity of theegress path A will now be described with regard to the magnetic fields Bproduced by currents in the antenna windings 24, 36 and 38.

In the successive 90 intervals of antenna energization represented byFIGS. 4A-D, each of the three antenna windings 24, 36 and 38is energizedin phase, according to the prior art. Thus all antenna windings conductmaximum current at the same time and all an-' tenna windings conductzero current at the next following 90 interval.

The major direction of magnetic field strength, which is represented byan arrow C in FIG. 4 is determined according to the mutual additive andsubtractive effects of the circular magnetic fields B in the variousantenna windings. As can be seen in FIG. 4A, the major magnetic fieldenergization C is at an angle a with respect to the path A. After thenext 90 interval the magnetic field energization diminishes to zero asshown in FIG. 4B. Thereafter as shown in FIG. 4C, the major magneticfield energization C is again at the angle a with respect to the egresspath A, but is reversed in direction. Finally, as shown in FIG. 4D,after the last interval the magnetic field strength again diminishes tozero.

It will be appreciated from FIG. 4 that the major magnetic fieldstrength C always lies along a path which crosses the egress path A atan angle (1. Also by considering the additive and subtractive effects ofthe circular magnetic fields B within the various quadrants (a), (b),(c) and (d), it will be seen that a minimum magnetic field strength isalways present in quadrants (b) and (d) while maximum magnetic fieldstrength is present only in quadrants (a) and ('6). Thus, by taking careto traverse the checkpoint along a path indicated by the dashed arrow D,one can cross the path of maximum field strength at substantially aright angle to it and thereby minimize the duration required to passthrough the high intensity region of the field. Also, by following thepath of the arrow D, one will traverse the checkpoint via the quadrants(b) and (d) of minimum field energization. These quadrants make up deadzones within which only minimal response can be obtained from arebroadcaster circuit. As a result a possibility exists in this priorart arrangement, for a protected article to pass through a checkpointundetected if it follows a particular path. Turning now to FIG. 5,wherein operation according to the present invention is shown, it willbe seen that the transverse antenna winding 24 is energized at a 90phase relationship to the energization of the lateral antennas 36 and38. As can be seen in FIG. 5A, when maximum current flows through thelateral antenna windings 36 and 38, no current flows through thetransverse antenna winding 24. The additive and subtractive effects ofthe circular magnetic fields B are such that they produce a net maximummagnetic field energization C which is at a right angle B to the egresspath A. After the next 90 interval, as seen in FIG. 5B, the currentthrough the lateral antenna windings 36 and 38 diminishes to zero andthe current through the transverse antenna winding 24 becomes maximum.The net effect is to swing the maximum magnetic field energization Cinto alignment with the egress path A. Thereafter, after the next 90interval of antenna energization, as shown in FIG. 5C, current in thetransverse antenna winding 24 diminishes to zero while current throughthe lateral antenna windings 36 and 38 again reaches maximum in theopposite direction from that shown in FIG. 5A. This causes the maximummagnetic field energization C to swing further around until it again isat a right angle B to the egress path A. Finally after the last 90interval of antenna energization, as seen in FIG. 5D, the circularmagnetic fields B cause the maximum magnetic field energization to swingback along the egress path direction A.

It will be appreciated that by interpolation it can be shown that themaximum magnetic field vector C revolves circularly during antennaenergization. Thus any dead zones are swept around and the entirevicinity of the egress path A is electromagnetically energizeduniformly. Further, since the electromagnetic field pattern movescontinuously there is no path along which one may pass in which themagnetic field strength is always minimal. Thus there is provided ameans for improving the detection reliability of responder circuits inelectronic theft detection system.

While the invention has been described with reference to the preferredform thereof, it will be obvious to those skilled in the art to whichthe invention pertains, after understanding the invention, that variouschanges and modifications may be made therein without departing from thespirit and scope of the invention, as defined by the claims appendedhereto.

What is claimed is:

1. In an article theft detection system the combination of at least twoantenna windings mounted in the vicinity of a checkpoint along anarticle egress path, said antenna windings lying in different planes, anoscillator for generating high frequency electrical signals, signaltransmission means connected between said oscillator and said antennawindings to energize said windings from said oscillator, said signaltransmission means including means forming separate signal paths to theantenna windings lying in the different planes and phase shift means inat least one of said signal paths for producing a relative phasedifference between the signals applied to the different antennawindings.

2. An article theft detection system according to claim 1 wherein theplanes of said two antenna windings are arranged traversely to eachother and wherein said phase shift means is operative to produce a netrelative phase difference of 90 between the signals applied to thedifferent antenna windings.

3. An article theft detection system according to claim 2 wherein atleast one antenna winding encircles said egress path and lies in a planeperpendicular to said egress path.

4. An article theft detection system according to claim 3 wherein twoadditional antenna windings lie in planes parallel to said egress path.

5. An article theft detection system according to claim 4 wherein saidone antenna winding is connected to be energized from one of said signalpaths and wherein both of said two additional antenna windings areconnected to be energized from another of said signal paths.

6. An article theft detection system according to claim 2 whereinseparate phase shifters are provided in said one and another signalpaths.

7. An article theft detection system according to claim 6 wherein one ofsaid separate phase shifters is operative to produce a leading phaseshift of approximately 45 and the other phase shifter is operative toproduce a lagging phase shift of approximately 45.

8. An article theft detection system according to claim 6 wherein saidphase shifters have complementary frequency sensitivity characteristicswhereby the same net phase difference is maintained at different sig nalfrequencies.

9. An article theft detection system according to claim 6 wherein onephase shifter comprises a resistor and a capacitor connected in parallelwith each other along one of said signal paths and said other phaseshifter comprises a resistor and an inductor connected in parallel witheach other along said another signal path.

10. Apparatus for generating an electromagnetic interrogation field at agiven checkpoint through which protected articles must pass, saidapparatus comprising at least two antenna windings lying in differentplanes in the vicinity of said checkpoint and means for applyingelectromagnetic signals of the same frequency but in phase shiftedrelationship simultaneously to said antennas.

11. Apparatus according to claim 10 wherein said antenna windings lie inplanes which are substantially perpendicular to each other and whereinsaid means for applying electromagnetic signals includes phase shiftermeans operative to maintain a substantially relative phase shift betweenthe signals applied to the antennas in different planes. I

12. An article theft detection system comprising a variable frequencyoscillator, a tuning oscillator connected to said variable frequencyoscillator for causing same to produce an output signal whose frequencyvaries cyclically at a rate corresponding to the frequency of saidtuning oscillator, means defining an output signal path including afirst junction connected to the output of said variable frequencyoscillator and means forming a pair of branch signal paths leading fromsaid first junction to associated antenna junctions, separate antennawindings connected respectively to said antenna junctions, said antennawindings being positioned at a given checkpoint and lying in differentplanes, detector means connected to said antenna junctions, saiddetector means being responsive to the changes in energy level at thejunctions which occur at a rate related to the frequency of said tuningoscillator to produce an output signal, alarm means arranged to beactuated by said output signal from said detection means and phaseshifter means arranged in at least one of said branch signal pathsbetween said first junction and one of said antenna junctions.

13. An Article Theft Detection System according to claim 12 wherein saidantenna windings include a first winding lying in a plane transverse toan egress path and a second winding lying in a plane extending laterallyof said egress path.

14. An article theft detection system according to claim 13 wherein saidphase shifter means is operative to produce a net phase difference insignals applied to said first and second windings of substantially 90.

15. An article theft detection system according to claim 12 wherein saidphase shifter means includes a pair of phase shifters each connectedalong a different one of said branch signal paths, one of said phaseshifters being operative to produce a leading phase shift and the otherbeing operative to produce a lagging phase shift.

16. An article theft detection system according to claim 15 wherein oneof said phase shifters comprises a resistor and a capacitor connected inparallel with each other along one of said branch signal paths and theother phase shifter comprises a resistor and an capacitor connected inparallel with each other along the other of said branch signal paths.

17. An Article Theft Detection System according to claim 12 where saiddetector means comprises separate detector devices connected,respectively, to each of said antenna junctions.

18. A method for generating an electromagnetic interrogation field at agiven checkpoint through which protected articles provided withelectronic responder circuits must pass, said method comprising the stepof electrically energizing antenna coils, positioned in different planesat said checkpoint, at the same frequency but at different phasesthereby to generate an electromagnetic field which rotates in thevicinity of the checkpoint.

19. A method according to claim 18 wherein said coils lie in planeswhich are mutually perpendicular and wherein said coils are energized 90out of phase with respect to each other.

1. In an article theft detection system the combination of at least twoantenna windings mounted in the vicinity of a checkpoint along anarticle egress path, said antenna windings lying in different planes, anoscillator for generating high frequency electrical signals, signaltransmission means connected between said oscillator and said antennawindings to energize said windings from said oscillator, said signaltransmission means including means forming separate signal paths to theantenna windings lying in the different planes and phase shift means inat least one of said signal paths for producing a relative phasedifference between the signals applied to the different antennawindings.
 2. An article theft detection system according to claim 1wherein the planes of said two antenna windings are arranged traverselyto each other and wherein said phase shift means is operative to producea net relative phase difference of 90* between the signals applied tothe different antenna windings.
 3. An article theft detection systemaccording to claim 2 wherein at least one antenna winding encircles saidegress path and lies in a plane perpendicular to said egress path.
 4. Anarticle theft detection system according to claim 3 wherein twoadditional antenna windings lie in planes parallel to said egress path.5. An article theft detection system according to claim 4 wherein saidone antenna winding is connected to be energized from one of said signalpaths and wherein both of said two additional antenna windings areconnected to be energized from another of said signal paths.
 6. Anarticle theft detection system according to claim 2 wherein separatephase shifters are provided in said one and another signal paths.
 7. Anarticle theft detection system according to claim 6 wherein one of saidseparate phase shifters is operative to produce a leading phase shift ofapproximately 45* and the other phase shifter is operative to produce alagging phase shift of approximately 45*.
 8. An article theft detectionSystem according to claim 6 wherein said phase shifters havecomplementary frequency sensitivity characteristics whereby the same netphase difference is maintained at different signal frequencies.
 9. Anarticle theft detection system according to claim 6 wherein one phaseshifter comprises a resistor and a capacitor connected in parallel witheach other along one of said signal paths and said other phase shiftercomprises a resistor and an inductor connected in parallel with eachother along said another signal path.
 10. Apparatus for generating anelectromagnetic interrogation field at a given checkpoint through whichprotected articles must pass, said apparatus comprising at least twoantenna windings lying in different planes in the vicinity of saidcheckpoint and means for applying electromagnetic signals of the samefrequency but in phase shifted relationship simultaneously to saidantennas.
 11. Apparatus according to claim 10 wherein said antennawindings lie in planes which are substantially perpendicular to eachother and wherein said means for applying electromagnetic signalsincludes phase shifter means operative to maintain a substantially 90*relative phase shift between the signals applied to the antennas indifferent planes.
 12. An article theft detection system comprising avariable frequency oscillator, a tuning oscillator connected to saidvariable frequency oscillator for causing same to produce an outputsignal whose frequency varies cyclically at a rate corresponding to thefrequency of said tuning oscillator, means defining an output signalpath including a first junction connected to the output of said variablefrequency oscillator and means forming a pair of branch signal pathsleading from said first junction to associated antenna junctions,separate antenna windings connected respectively to said antennajunctions, said antenna windings being positioned at a given checkpointand lying in different planes, detector means connected to said antennajunctions, said detector means being responsive to the changes in energylevel at the junctions which occur at a rate related to the frequency ofsaid tuning oscillator to produce an output signal, alarm means arrangedto be actuated by said output signal from said detection means and phaseshifter means arranged in at least one of said branch signal pathsbetween said first junction and one of said antenna junctions.
 13. AnArticle Theft Detection System according to claim 12 wherein saidantenna windings include a first winding lying in a plane transverse toan egress path and a second winding lying in a plane extending laterallyof said egress path.
 14. An article theft detection system according toclaim 13 wherein said phase shifter means is operative to produce a netphase difference in signals applied to said first and second windings ofsubstantially 90*.
 15. An article theft detection system according toclaim 12 wherein said phase shifter means includes a pair of phaseshifters each connected along a different one of said branch signalpaths, one of said phase shifters being operative to produce a leadingphase shift and the other being operative to produce a lagging phaseshift.
 16. An article theft detection system according to claim 15wherein one of said phase shifters comprises a resistor and a capacitorconnected in parallel with each other along one of said branch signalpaths and the other phase shifter comprises a resistor and an capacitorconnected in parallel with each other along the other of said branchsignal paths.
 17. An Article Theft Detection System according to claim12 where said detector means comprises separate detector devicesconnected, respectively, to each of said antenna junctions.
 18. A methodfor generating an electromagnetic interrogation field at a givencheckpoint through which protected articles provided with electronicresponder circuits must pass, said method comprising the step ofelectrically energizing antEnna coils, positioned in different planes atsaid checkpoint, at the same frequency but at different phases therebyto generate an electromagnetic field which rotates in the vicinity ofthe checkpoint.
 19. A method according to claim 18 wherein said coilslie in planes which are mutually perpendicular and wherein said coilsare energized 90* out of phase with respect to each other.